12 



RADIATION BIOLOGY 



a 



(see Fig. l-9a) provided that the oscillations are timed to apply the acceleration 

 in the correct direction whenever a group of particles arrives at the gap. An 

 arrangement of this kind with a large number of gaps in a straight line constitutes 

 "linear accelerator." 



However, most frequently the particles are made to return again and again to 



the same gap by forcing them to follow a closed path by 

 means of a deflecting magnet. A machine built on this 

 principle, in which the deflecting field remains constant 

 and the particles turn in larger and larger circles as they 

 gather momentum, is called a "cyclotron" (Fig. 1-10). 

 A "synchrotron" is a machine in which the particles are 

 kept on a constant path by a magnetic field whose 

 strength increases as the particles gather momentum. A 

 number of combinations of these principles have been 

 developed under various names. 



Another type of machine relies on the principle of the 

 familiar a-c transformer and is called an "induction 

 accelerator" or a "betatron" (because it is particularly 

 suited to the acceleration of electrons, i.e., of |3 particles). 

 A transformer consists of an electromagnet excited by an 

 alternating current in a primary winding. The alternat- 

 ing magnetic force induces, in turn, a current in another 

 coil, the secondary winding. The voltage developed at 

 the terminals of the secondary is proportional to the 

 number of turns in the winding. A betatron is, in effect, 

 a large transformer whose secondary current consists of 

 electrons circulating in an evacuated ring-shaped tube, 

 called the " donut " (see Fig. 1-11). Each electron circulates a very large number 

 of times (e.g., 1,000,000 times) and thus gathers an energy corresponding to the 

 voltage that would be achieved in a secondary winding with a similarly large 

 number of turns. 



CCD dZD 



Fig. 1-8. Diagram of 

 an accelerator tube. 

 {Charleton and Hub- 

 hard, 1940.) 



^ 



(a) (b) 



Fig. 1-9. Diagram of the elements of a linear accelerator, (b, Chew and Moyer, 1950.) 



The main obstacle to the development of high-performance linear 

 accelerators is found in timing accurately the voltage oscillations at many 

 different gaps. The necessary length of the machine and the problems 

 of keeping the particles in a focused beam also cause difficulties. The 

 main limitation in the development of cyclotrons and synchrotrons 

 arises from the necessity of maintaining very-high-energy particles on a 

 closed steady path. The deflection of the particles requires very large 



